Depends on the scooter in some cases. A Honda Reflex 250 project I am back to working on again has a taller Polini gear set installed. (Malossi has an even taller gear set for the Reflex) It also has a much larger than stock passenger vehicle tire (car tire) installed as well. All with the aim of lowering engine rpms.
But that is not without some drawback. I actually need to use a different car tire that is a bit shorter (I have one, just not installed yet) or go back to the standard final drive gears. The test ride I did with both the extra tall tire and taller gears had the engine lugging... So, too much of a good thing not being too good really.
Wheels vary between scooters as well. (though still shorter than conventional motorcycle wheels in most cases) Honda Helix(144cc) has 10" wheels - Reflex(249cc) 12"R, 13"F - Silverwing 600 13"R, 14"F - Suzuki Burgman 650 14"R, 15"F for examples.
For an idea of what can be done, check into this:
Craig Vetter Fuel Economy Challenge. better mileage. Living better on less energy